Process for preparing sulphur hexafluoride

ABSTRACT

Disclosed is a process for the production of sulphur hexafluoride comprising the steps of reacting fluorine and molten sulphur, removing lower-boiling by-products, separating a portion of the sulphur hexafluoride from higher-boiling by-products and treating the residue containing the higher-boiling by-products at a temperature between about 450° and 800° C. for a period of time between about 0.1 and 25 seconds. The treatment step may be carried out in the presence of elementary fluorine, and provision may be made for recycling the treated or untreated residue to the reaction stage. An apparatus for carrying out the process is also disclosed.

This is a division of application Ser. No. 550,159 filed Feb. 14, 1975,now U.S. Pat. No. 4,039,646.

BACKGROUND OF THE INVENTION

The present invention relates to the preparation of sulphurhexafluoride, and more especially, to an improved process and apparatusfor the production of sulphur hexafluoride from elementary fluorine andsulphur, with the improvement comprising the elimination and/or recycleof the higher-boiling by-products produced during industrial productionprocesses of this type.

The industrial production of sulphur hexafluoride is carried out bymeans of a reaction of elementary fluorine with molten sulphur. Theelementary fluorine is produced by electrolysis of hydrogen fluoride,and always contains -- dependent upon the process of production --impurities of hydrogen fluoride, oxygen, nitrogen as well asperfluorinated alkanes and cycloalkanes. Furthermore, depending upontrace impurities of the hydrogen fluoride, the fluorine may alsocontain, for example, the compounds OF₂ (from H₂ O) and ClO₃ F (fromchlorine compounds).

In addition to lower sulphur fluorides such as S₂ F₂, SF₄ and S₂ F₁₀,the sulphur hexafluoride produced by the reaction of fluorine withmolten sulphur can also contain a number of further impurities, forexample, SF₅ OS₅ ; SF₅ OCF₃ ; SOF₂ ; SOF₄, SO₂ F₂ ; C_(n) F_(2n+2) ;C_(n) F_(2n) ; ClO₃ F as well as oxygen O₂ and nitrogen N₂. The rawsulphur hexafluoride SF₆ so produced must therefore be purified, inorder to fulfill the stringent purity requirements which apply to theuse of SF₆ as an insulating and quenching gas.

Normally, the highly toxic S₂ F₁₀ is removed from the product by meansof a down-stream temperature treatment at a temperature of approximately400° C, in accordance with the equation:

    S.sub.2 F.sub.10 → SF.sub.4 + SF.sub.6

similarly, hydrolyzable compounds such as SF₄, S₂ F₂, SOF₂, SOF₄ and SO₂F₂ are removed by means of a subsequently carried out alkali wash and/oralkali absorption step.

Finally, the lower-boiling components such as O₂, N₂ and CF₄ areremoved, for example, by being stripped in a first stage of a two-stagepressure distillation step, and the higher-boiling components areconcentrated in the column sump of the second distillation stage.

This column sump represents a mixture of different materials andcontains, in addition to the principal component SF₆, the higher-boilingimpurities, especially SF₅ OSF₅, CF₃ OSF₅, ClO₃ F, perfluorinatedalkanes and cycloalkanes. Several of these materials are highly toxic.There exists also the possibility that, in the case of any failure ortrouble, e.g., with loss of the temperature treatment or as a result ofincomplete washing, other materials may also pass into the column sump,which normally are removed in the upstream purification stages.

The column sump composition is thus a natural waste product, theemptying and storage of which is fraught with risks. As a result of theheterogeneous composition of the waste product, chemical destruction tosafe products is problematic, difficult and expensive.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved process for the production of sulphur hexafluoride.

It is a further object of the invention to provide an improved processfor the production of sulphur hexafluoride wherein the SF₆ wastematerial containing higher-boiling impurities is safely and easilydisposed of.

A further object of the invention resides in the provision of animproved process for the production of SF₆ wherein the higher-boilingimpurity-containing SF₆ is recycled to the reaction step to increase theyield of SF₆ and decrease the production of by-products.

Another object of the invention resides in the provision of an apparatusfor carrying out the improved process of the invention.

In accomplishing the foregoing objects, there has been provided, inaccordance with the present invention, an improved process for theproduction of sulphur hexafluoride SF₆ comprising the steps of reactingfluorine and molten sulphur, whereby a mixture of SF₆ with higher- andlower-boiling by-products is produced, removing the lower-boilingby-products from the mixture, separating a portion of the SF₆ from thehigher-boiling by-products, whereby a residue of SF₆ containing a minorproportion of higher-boiling by-products results, and treating theresidue at a temperature between about 450° and 800° C., preferablybetween about 500° and 650° C., for a period of time between about 0.1and 25 seconds, preferably between about 0.5 and 5 seconds, optionallyin the presence of elementary fluorine. The process may further includethe step of contacting the thus-treated residue with an aqueous alkalisolution to remove fluoro-sulphur compounds, and advantageously, thetreated residue may be recycled back to the reaction step, preferablyintroducing same into a supply conduit for fluorine to the reaction.Recycling may be carried out continuously or intermittently, i.e.,batchwise.

Typically, the step of separating a portion of the SF₆ from thehigher-boiling by-products comprises a distillation step producing theresidue as a bottoms-product, and in this case the process is furthercharacterized by the step of vaporizing the bottoms-product prior tocarrying out the described treatment step.

In another aspect of the present invention, there is provided a processwhich includes a step of recycling the residue directly back to thereaction step, whereby the treating step is carried out concurrentlywith the reaction step. Where this direct recycle is carried out, theprocess also inherently includes the step of contacting thetemperature-treated residue with an aqueous alkali solution, by virtueof the fact that the temperature-treated residue passes into an aqueousalkali solution contact step subsequent to the reaction step.

In a further aspect of the invention, there has been provided anapparatus for the production of sulphur hexafluoride SF₆ from fluorineand molten sulphur, comprising:

(a) a vessel for reacting fluorine and molten sulphur, wherein a mixtureof SF₆ together with higher- and lower-boiling by-products is produced;

(b) means such as an alkali wash and/or pressure still after thereaction vessel for removing the lower-boiling by-products from themixture;

(c) means such as a pressure distillation apparatus connected with theabove apparatus for separating a portion of the SF₆ from thehigher-boiling by-products, wherein a residue of SF₆ containing a minorproportion of higher-boiling by-products is produced; and

(d) means such as a heated chamber or a recycle back to the reactionvessel for treating the residue at a temperature between about 450° and800° C.

Further objects, features and advantages of the present invention willbecome apparent from the following detailed description of severalspecific embodiments, when considered together with the accompanyingfigures of drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic diagram of an apparatus suitable for carrying outthe process of the invention; and

FIG. 2 is a schematic diagram of a modified apparatus in accordance withthe invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

In accordance with the present invention, there has been discovered aprocess which permits a batchwise or continuous working up of the columnsump to eliminate non-toxic and easily separable components. In apreferred embodiment of the invention, the working up of the sumpmaterial is carried out in connection with the sulphur hexafluorideproduction process by recycling the sump into the reaction and/orpurification part of the process.

It has been discovered, in particular, that a treatment of the sump at atemperature greater than 400° C, typically between about 450° and 800°C, and preferably between about 500° and 650° C, decomposes the SF₅OSF₅, CF₃ OSF₅ and ClO₃ F impurities, whereas the principal componentSF₆ is still not, or only to a small degree, affected at thesetemperatures. Some of the higher fluorocarbon compounds, such as,perfluoromethylcyclopentane, are partially decomposed, and otherfluorocarbons, such as perfluoronated-N-alkanes, remain largelyunaffected. Any S₂ F₁₀ present is decomposed already at temperatures of400° C, as is known.

If the gaseous product coming from the temperature treatment step iswashed with an alkali solution, e.g., potash lye, it will not containany more fluoro-sulphur compounds other than SF₆ and some fluorocarboncompounds.

It has also been discovered that the higher molecular weightfluorocarbon compounds are largely broken down into lower compounds,predominantly into CF₄, by the addition of elementary fluorine duringthe temperature treating step using a volumetric relationship of 0.1 to2 parts of the residue respectively therefor to 1 part of fluorine andpreferably 1 part residue to 1 part fluorine.

At the same time, the sulphur-containing products are fluorinated toSF₆. Such a decomposition is also observed if the temperature treatmentstep is carried out without and with elementary fluorine, respectively,in two reactors connected in series.

A considerable decrease in the proportion of higher-boilingfluoro-sulphur compounds and C10₃ F as well as of the proportion ofhigher-boiling perfluorinated alkanes and cycloalkanes in the columnsump can also be accomplished by vaporizing the bottoms product of thecolumn and adding the gas mixture to the fluorine supply conduit whichleads to the SF₆ reactor. Under the reaction conditions of the SF₆synthesis, in connection with which temperatures of greater than 450° C.occur in the gas phase, obviously reactions take place in the same wayas those which are achieved by a combination of temperature treatmentwithout and with fluorine.

A preferred industrial embodiment of the process is represented withreference to the schematic process illustration of a SF₆ facility suchas disclosed in Chemiker Zeitung, 96, (1972), page 73, which is shown inFIG. 1 of the drawings. The SF₆ production process operates inconventional manner, wherein molten sulphur is introduced via a supplyline 10 and elementary fluorine via supply line 12 into the reactionvessel 14. The gaseous reaction product is removed from the reactionvessel and conveyed to a pyrolysis vessel 16 wherein temperatures of400° C. or more are produced in order to remove S₂ F₁₀. After pyrolysis,the gaseous mixture is conveyed to a washing device 18 where it iscontacted with an aqueous alkali solution, such as NaOH. Upon leavingthe washing step, and after having been dried in absorption-drying unit20, the gaseous mixture is compressed by compressor 22 and introducedinto a 2-stage compression distillation stage carried out in the twodistillation columns identified with reference numerals 1 and 2. Column1 is a stripper column wherein lower-boiling, and inert gases such asO₂, N₂ and CF₄ are removed. Substantially pure SF₆ is removed from thetop of column 2, whereas the higher-boiling impurities are concentratedin the sump of this column.

In accordance with the improved process of the present invention, thebottoms product of column 2 is continuously released via the line 3 andthe dosing valve 4. After passing through the vaporizer 5, the gases arefed through the temperature treating device 6 and finally via line 7into the fluorine supply line 12 which leads to reactor 14 in the SF₆facility.

By virtue of this method of operation, the result is achieved that theSF₆ contained in the column sump is not lost, that the hydrolyzabledecomposition products obtained in the temperature treatment stage areseparated together with the hydrolyzable products produced in the SF₆synthesis in the washing device of the SF₆ production apparatus or arefluorinated to SF₆, and that finally, a substantial fluorination of theperfluorinated alkanes and cycloalkanes into CF₄ also results. The CF₄together with the CF₄ present in the F₂, is separated in the strippercolumn 1 of the pressure distillation stage.

A further industrial embodiment of the process involves the directrecycle of the gaseous bottoms product exiting from the vaporizer 5 backto the fluorine feed stream entering reactor 14, i.e., without previoustemperature treatment. This embodiment of the invention can be carriedout with the apparatus illustrated in FIG. 1 by providing a by-pass line8 around the temperature treatment chamber 6.

Both embodiments of the process guarantee that the proportion ofhigher-boiling products present in the sump can be held at a low-level,that no difficulty separable and difficulty removable waste products areproduced and that no loss of SF₆ can take place.

It is also known that the possibility exists to separate thehigher-boiling fraction present in the SF₆ by treatment of the impureSF₆ with absorbents, such as, for example, activated carbon or molecularsieves, instead of a pressure distillation technique. When theabsorbents are regenerated, which normally is accomplished by heating,this high-boiling fraction together with the concurrently absorbed SF₆is set free. In an alternative embodiment of the present invention, theSF₆ production process can be carried out utilizing such an absorptionstage, and the steps provided according to the invention for treatingand removing the mixture of impurities may be carried out in conjunctionwith such a modified process. The apparatus for carrying out thisalternative embodiment is illustrated in FIG. 2 of the drawings whereinan absorption unit 25 is provided in the chain of apparatus in place ofthe second distillation column 2 in FIG. 1. The absorbent from theabsorption unit is regenerated in a regeneration stage 26 provided inthe recycle loop to the reactor 14, which recycle loop containstemperature treatment chamber 6.

The treated gas mixture may be washed with alkali solutions of variousbases and concentrations. Preferably a 5 to 10 percent solution ofpotassium hydroxide is used in order to avoid precipitates and muds.

The following examples are presented to more fully describe the presentinvention, it being understood that the examples are intended to bemerely illustrative and in no sense limitative.

EXAMPLE 1

A gas sample from the distillation sump of a SF₆ production unit isconducted at a temperature of 600° C. through a stainless steel tubehaving an inside diameter of 15 mm. and a length of 1200 mm. The tube isprovided with an electrical heating coil having a length ofapproximately 1000 mm.

The gaseous sample from the distillation column has the followingcomposition as determined by gas chromatography and infraredspectroscopy (in percentage of area of the gas chromatogram):

Peak 1--SF₆ : 76.2%

Peak 2--i-C₄ F₁₀ ; n-C₄ F₁₀ ; CF₃.O.SF₅ : 3.6%

Peak 3--ClO₃ F; n-C₅ F₁₂ : 0.5%

Peak 4--C₅ F₁₀ (Perfluorocyclopentane): 8.4%

Peak 5--SF₅ .O.SF.sub. 5 ; n-C₆ F₁₄ : 6.7%

Peak 6--C₅ F₉ -CF₃ (Perfluoromethylcyclopentane); S₂ F₁₀ : 3.4%

Peak 7--C₆ F₁₂ (Perfluorocyclohexane): 1.1%

Peak 8--Higher perfluoronated hydrocarbon compounds: 0.2%

A gas residence time of 40 seconds is chosen. The gas composition whichis subjected to the temperature treatment is conducted through a 10%potash lye solution and is thereafter led through a gas receiver fromwhich samples are taken for analysis.

The gas chromatogram shows a reduction in the area of peaks 2, 3, 5 and6. The IR analysis results in a finding that the compounds CF₃ OSF₅ ;ClO₃ F; SF₅ OSF₅ and S₂ F₁₀ are no longer detectable.

EXAMPLE 2

A distillation bottoms product having the composition specified inexample 1 is subjected to a temperature treatment at 500° C. with aresidence time of 100 seconds under otherwise similar conditions. Theresults of the analysis are the same as in example 1, with the exceptionthat the treated gas still contains approximately 10% of the initialconcentration of the component SF₅ OSF₅.

EXAMPLE 3

A sample of the distillation bottoms product used in Example 1 istreated in admixture with fluorine in a volumetric relationship of 1:1at a temperature of 500° C. and with a residence time of 10 seconds.Thereafter, the sample is diluted with nitrogen and the resultingmixture is directed into a thoroughly cooled washing flask with a 10%aqueous potash lye solution. The non-hydrolyzable gases are collected ina gasometer and analyzed.

In addition to N₂ and SF₆, CF₄, C₂ F₆, SO₂ F₂ and O₂ (from the reactionof fluorine with potash lye) are detected.

EXAMPLE 4

In a device, which is installed in a SF₆ production unit and which isschematically illustrated in FIG. 1 of the drawings, liquid in an amountof 1 kg/hr is withdrawn via a dosing valve from the still bottom of aSF₆ distillation column and is expanded to a pressure of about 1 bar.After passing through a vaporizer, which is maintained at a temperatureof 100° C., the now gaseous material passes through a tube of stainlesssteel (inside diameter: 30 mm., heated length: 1500 mm.). The gastreated in this manner is recycled into the fluorine supply line,shortly before its entrance into the reactor. The conditions chosencorrespond to a gas residence time of around 24 seconds.

During the operation of the device for the temperature treatment,samples are taken from the bottom of the distillation column at regularintervals for analysis. The results of the analysis show a steadyreduction of the concentration of the total content of higher-boilingby-products. Parallel to this finding is a finding that an increasedcontent of CF₄ is found in the inert gases withdrawn at the top of thefirst distillation column.

During the duration of the experiment, there resulted a steady, butvaried decrease in the concentration of the individual components in thebottoms product of the distillation column, and in this regard theconcentration of the sulphur-containing compounds decreases to a greaterextent. Thus, for example, a C₅ F₁₀ content of 5.73% decreased in thecourse of several hours to only 4.99%, whereas the content of SF₅ OSF₅decreased from 1.36% to 0.02%.

EXAMPLE 5

From the bottom of the distillation column in a SF₆ production unit,approximately 2 kg/hr of the bottoms product is removed via a dosingvalve and expanded to a pressure of about 1 bar. The gas mixtureobtained as a result is introduced into the fluorine supply line priorto the sulphur reactor. In this manner, the gas is brought into contactwith fluorine in the reactor, where it is partially converted, and thenthe gas continues on over the wash system and into the distillationcolumns.

Based upon the analysis samples, which are removed from the bottom ofthe distillation column at regular intervals of time, there is evidenceda progressive decrease in the concentration of all components, of whichthe sulphur-containing compounds are above all affected.

Already after several hours of operating the recycle, the proportion ofsulphur-containing compounds in the distillation sump is reduced by50-90% of the initial content.

What is claimed is:
 1. In a process for the production of sulphurhexafluoride SF₆ including the steps of reacting fluorine and moltensulphur, whereby a mixture of SF₆ together with higher- andlower-boiling by-products is produced, removing said lower-boilingby-products from said mixture, and separating a portion of said SF₆ fromsaid higher boiling by products, whereby a residue of said SF₆containing a minor portion of higher-boiling by-products results, theimprovement comprising the step of recycling said residue back to saidreacting step.
 2. The process as defined in claim 1 wherein prior tosaid recycling step, said residue is treated by pyrolyzing in thepresence of 1 part elementary fluorine for each 0.1 to 2 parts residue.3. The process as defined by claim 1, further comprising the step ofpyrolyzing said residue at a temperature between about 450° and 800° Cfor a period of time between about 0.1 and 100 seconds before recyclingsaid residue back to said reacting step.
 4. The process as defined byclaim 1, wherein said treating step is in the presence of one part ofsaid residue to one part of said fluorine.
 5. The process as defined byclaim 1, further comprising the step of contacting said treated residuewith an aqueous alkali solution, whereby fluoro-sulphur compounds areremoved from said product.
 6. The process as defined by claim 1, whereinsaid treating step is carried out at a temperature between about 500°and 650° C.
 7. The process as defined by claim 1, wherein said treatingstep is carried out for a period of between about 0.5 and 5 seconds. 8.The process as defined by claim 1, wherein said treated residue isrecycled continuously to said reaction step.
 9. The process as definedby claim 1, wherein said treated residue is recycled intermittently tosaid reaction step.